1. Field of the Invention
The present invention relates to a laser welding apparatus and in particular, to a laser welding apparatus for welding in a shield gas atmosphere.
2. Description of the Related Art
FIG. 13 shows a conventional laser welding apparatus 100. The laser welding apparatus 100 includes a laser output mechanism 101 for emitting a welding laser beam A from its output end 101a. This laser output mechanism 101 has an optical system (not depicted) for setting a focal point of the welding laser beam A in the laser emission direction. The reference symbol 101b denotes a transparent protection plate for protecting the optical system from sputter during welding.
Furthermore, this laser welding apparatus 100 includes a jet unit 102 for spraying in jet a shield gas C for preventing oxidization of an object to be welded B during welding. This gas jet unit 102 is a cylindrical member having a laser output mechanism 101 inserted therein. The shield gas C is sprayed passing through a space between the inner wall of the gas jet unit 102 and the laser output mechanism 101 in the direction identical to the laser beam A emission.
When the laser welding apparatus 100 having the aforementioned configuration applies the welding laser beam A to an object B to be welded, a metal plasma D is generated. Simultaneously with this, the shield gas C is sprayed in jet so that welding of the object B is performed isolated from the oxygen of the atmosphere.
FIG. 14 shows another conventional laser welding apparatus 100A. This laser welding apparatus 100A, instead of the cylindrical gas jet unit 102, includes a jet nozzle 102A for spraying the shield gas C in one direction toward the welding position. The shield gas C is applied in jet almost in a vertical direction with respect to the laser beam A emission.
FIG. 15 shows still another conventional laser welding apparatus 100B. In comparison to the laser welding apparatus 100A, this laser welding apparatus 100B includes: a laser output mechanism 101B not having the protection plate 101b; and an air nozzle 103B for spraying a compressed air toward the output tip of the laser output mechanism 101B. This air nozzle 103B protects the output end of the laser output mechanism 101B from sputter.
However, in the aforementioned conventional laser welding apparatuses, the shield gas C is blown so as to blow off the metal plasma D generated during laser welding, from the welding position. No consideration has been taken on the affect to the welding range.
That is, in the laser welding apparatus 100, the welding laser beam A is applied together with the shield gas C, and the metal plasma D generated in the welding position is disturbed as shown in FIG. 13. Moreover, in the other laser welding apparatuses, the shield gas C is applied from one direction, and accordingly, the metal plasma D is pushed toward the downstream and cannot be stay at the welding position.
Here, explanation will be given on the relationship between the metal plasma and the laser welding. Conventionally, it has been considered that the plasma generated during laser welding lowers transmittance of the welding laser beam, preventing effective transmittance of laser energy to an object to be welded. However, it has been observed as test results that when the metal plasma is moved by the shield gas, the melting capability of the metal to be welded is lowered.
FIG. 16 and FIG. 17 show experimental data, i.e., the difference between melting amount when the shield gas is supplied and when not supplied. In the experiment, a metal sample made from aluminum was subjected to the welding laser beam while fluctuating an energy density per a unit welding length with and without shield gas blowing. The width and depth of the melting were measured. Here, the shield gas was blown from the same direction as the aforementioned conventional laser welding apparatus 100. The amount of the shield gas was 20 litters and the nozzle diameter was 5 mm.
FIG. 16 has a horizontal axis indicating an energy density per a unit welding length and a vertical axis showing the welding depth change caused by the energy density change.
FIG. 17 has a horizontal axis indicating an energy density per a unit welding length and a vertical axis showing the welding range (width) change caused by the energy density change.
In both of FIG. 16 and FIG. 17, the solid line indicates the value obtained when the shield gas was used and the broken line shows values obtained when no shield gas was used.
FIG. 16 shows that the melting depth is increased when no shield gas is supplied. FIG. 17 shows that the melting width is slightly increased when no shield gas is supplied.
Thus, in the conventional examples, the metal plasma growth is deteriorated, disabling to obtain a sufficient welding depth and width and sufficient welding strength.